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Enlarging Perspectives on Space (and Time)

What do we mean by an ‘interstellar mission’? The question came up in relation to Interstellar Probe, that ‘Voyager Plus’ concept being investigated by the Johns Hopkins Applied Physics Laboratory. I do indeed see it as an interstellar mission, as Interstellar Probe takes us outside the heliosphere and into the local interstellar medium. We need to understand conditions there because it would be folly to mount a mission to another star without knowing the dynamics of the heliosphere’s movement through the interstellar cloud we are currently in, or the ramifications of moving between it and the adjacent cloud as we make our crossing.

How could it be otherwise? Journeys need maps and knowledge of conditions along the way. Thus we push into the fringes of interstellar space, and gradually extend our reach. As we do this, we inevitably produce changes in the way we perceive our place in the cosmos.

Cultural expectations about space have been shaped by what I might call a ‘planar’ approach to astronomy. First there is the Moon, then Mars, then the main asteroid belt, and so on, all of these things at increasing distances but roughly along the great disk of the ecliptic. In the 1950s science fiction film Rocketship X-M, a Moon mission misses its target through a series of odd misadventures and winds up landing on Mars. It was entertaining in its way as Lloyd Bridges and team explored the Red Planet, but it depicts a view of the Solar System in which if you go one distance, you’re at one target, and if you go another, you’re at the next. Never mind that the rocket’s mishap was entirely random and it could have gone anywhere.

Long-period comets and odd objects like Sedna teach us much about what goes on outside the ecliptic, but most deep space missions that have commanded the public’s attention have had destinations somewhere within it. The two Voyagers have a more complicated story given their gravitational encounters, Voyager 1 having taken a jog at Saturn to fly by Titan and thus propel itself out of the ecliptic on an interstellar trajectory, while its twin, Voyager 2, left the system and ecliptic in another direction after its encounter with Neptune. Neither was designed for interstellar operations but both now comprise our only live craft beyond the heliosphere.

As our missions become still more ambitious, we push into this wider, spherical realm of reference, which inevitably shapes public attitudes about our relationship with the galaxy. New Horizons’ mission to Pluto reminds us that the dwarf planet is at a 17° tilt to the ecliptic. Going to other stars would shed this culturally embedded planar concept, for the most part, though it’s interesting that one nearby destination, Epsilon Eridani, lines up well enough with the ecliptic to offer a boost from the angular momentum available to a departing craft. Alpha Centauri, well south of the ecliptic, demands a trajectory bend that loses this bit of assistance. This is a point APL’s Ralph McNutt made to me almost 20 years ago, as I was reminded recently in going through my notes from that period.

Image: Voyager 1 and 2 trajectories. Voyager 1 visited Jupiter and Saturn, and then veered northward off of the plane of our solar system. Voyager 2 visited all four giant planets of the outer solar system before departing southward toward interstellar space. Credit: NASA.

When we start contemplating interstellar missions, we have the chance to do what Voyager did just once, to look back at the Solar System, but this time in a much broader context. The focus will not be on the planets and the pale blue dot of Earth, but rather on the heliosphere, from a vantage well beyond its outer regions. Interstellar Probe is a heliophysics mission in its attempt to understand the Sun and planets as a system moving through the interstellar medium. It pushes perspectives as we visualize the entire Solar System as a moving, interacting environment where life can emerge.

The burgeoning catalog of exoplanets clearly plays into the concept, for we see thousands of stellar systems, each with their own context in what we can call an ‘astrosphere.’ The host stars we study, a tiny fraction of the several hundred billion in the galaxy, all move through plasma and dust within the interstellar medium. We have little enough information about how the Sun’s solar wind carves out the magnetic bubble surrounding our Solar System, but about astrospheres around other stars, we know next to nothing. Our view is flattened; we see their planets, or their circumstellar disks, our instrumentation not up to the challenge of seeing an astrosphere.

Image: This is Figure 3-1 from the JHU/APL report on Interstellar Probe from 2019; the latest report will be out in December. Caption: As our type-G2V star plows through the galactic interstellar medium, it forms the habitable astrosphere harboring the entire solar system we live in. Of all other astrospheres, one of our habitable type has never been observed, and yet we are only at the very beginning of uncovering our own. An interstellar probe through the heliospheric boundary into the LISM would enable us to capture its global nature and would represent humanity’s first step into the galaxy, where unpredictable discoveries await. Credit: NASA/Rosine Lallement, 2020.

Make no mistake, the crossing of the heliopause by both Voyagers has supplied us with data on the plasma physics at work in this region, while from inside the heliosphere, missions like IBEX have revealed unusual features that demand clarification. Interactions at heliosphere’s edge involve solar plasma, and magnetic fields both solar and interstellar, as well as neutral particles in the medium and galactic cosmic rays. Charge-exchange processes between interstellar hydrogen atoms and solar plasma protons shape the heliosphere as does the solar magnetic field pervading it.

A mission that gets to a vantage as distant as 1000 AU will be able to see these interactions from the outside, to determine the heliosphere’s overall shape and the distribution of plasma within it, even as missions like the upcoming IMAP (Interstellar Mapping and Acceleration Probe) study the heliosphere’s boundary from well within it. A probe into the interstellar medium would allow us to examine how the Sun’s activity cycle affects the heliosphere’s recorded shock and pressure waves, as found in Voyager data. Voyager has also shown that the heliosphere shields the Solar System from approximately 75 percent of incoming galactic cosmic rays, a factor in habitability.

But back to movement through the medium. Many interstellar clouds are found in what is called the Local Bubble,a region of hot gas that extends several hundred light years from the Sun. The conception of the Solar System as moving through interstellar clouds of varying dust, plasma and gas content backs out the field of view yet again. The Sun moves at 26 kilometers per second toward the edge of the Local Interstellar Cloud and will exit it in about 1900 years, and the question of what cloud we move through next is open. Fifteen interstellar clouds have been identified within 15 parsecs of our system.

Our Voyagers will run out of power somewhere in the range of 160 AU from the Sun, a long way from what astronomers consider the undisturbed local interstellar medium. Putting a probe well beyond this range would provide the first sampling of the interstellar medium that is unaffected by the heliosphere, and thus teach us a great deal about what our solar bubble moves through. As interstellar dust grains are the foundation of both stellar and planetary systems, they hold clues to the formation of matter in the galaxy and the evolution of stars. All this is applicable, of course, not just to our own heliosphere but the astrospheres around exoplanetary systems.

Image: This is Figure 3-10 from the JHU/APL report. Caption: The Sun is on the way to exiting the Local Interstellar Cloud and entering another unexplored interstellar region. Credit: NASA/Goddard/Adler/U. Chicago/Wesleyan.

A mission designed to be returning data 50 years after launch, expressly interstellar in its conception, also elevates our thinking about time as we confront operations long after our own demise. Such a mission puts the blip of our present existence into the context of galactic rotation, the chronological equivalent of the pale blue dot image.

Deeper awareness of ourselves as part of a great astrophysical complex that renders life possible helps to place us in a galactic setting. Going interstellar demands looking a long way out, but it also demands looking back, in our data and imagery, to understand the bubble within which we emerged. That shift in perspective in turn feeds the interstellar ambition, as we expand the frame of reference to other stars.


Comments on this entry are closed.

  • Alex Tolley October 13, 2021, 9:32

    Perhaps we need another suitcase word for missions that enter into the medium between stars.

    When we use “inter” on Earth; intercity, intercontinental we me travel from one city or continent to another. We don’t mean stop off at the edge of the city or continent. The use of “inter” to also mean “between”, as in interstellar medium, seems to me unfortunately used with the meaning of travel from one object to another. Using the terrestrial travel analogy and use of “inter”, an interstellar mission should mean travel from one star system to another. But here we are now using it to mean travel within a space. We use air travel, or sea travel to mean traveling with the medium without assuming traveling from one place to another. Voyager is really now on an “interstellar medium” mission, not an “interstellar mission”.

    If only we had a better term for the ISM that defines the medium rather than its relation to other objects – stars. We don’t say “intercontinental medium” to describe the ocean or the air, as we have terms to describe those media. We don’t have the appropriate term for the ISM. As we learn more about it, perhaps we can provide a new term for it as an entity, rather than its relationship to other entities.

    • George King October 13, 2021, 21:14

      The “interstellar mission” nomenclature has been mildly bugging me grammatically for the reasons that you state.

      I know that the phrase has different connotations in sundry science fiction, but “the expanse” perhaps has that same all encompassing and all-tying-togetherness sense as our references to “the ocean” on Earth.

      One then can distinguish as to a galactic (or intragalactic) expanse as opposed to, for example, an intergalactic expanse.

      And then perhaps rename this mission the Galactic Expanse Explorer, or GXX.

      As this will be one of our first exploration missions of the galactic expanse further out from where it figuratively laps up on our solar system’s shore at the outer edge of the heliosphere.

      Perhaps stirring the imagination as to what the exploration truly entails — a tentative initial exploration of the (local area of the) expanse that connects our entire galaxy, as opposed to “merely” the medium between stars in isolation.

      Referring to “a voyage into the galactic expanse” perhaps captures the broader perspective to which Paul refers in his piece. Even if we’re not going all that far into it with this mission.


      Personally, my temptation would be to focus now first on the materials research required to take either reflective and/or diffractive light sail technology to the next level. I.e., to develop the disruptive propulsion technology that potentially might achieve well in excess of 30km/s.

      That of course wouldn’t fit the 2030’s launch timeline in their analysis. But if we are taking a longer range view . . . .

      However, taking into account funding realities and the difficulty of at least modern governments to focus on coordinating exceedingly long range projects, it’s hard to be dismissive of this mission if NASA is inclined to pursue this exploration now (for a 2030’s launch), seguing from the final wind down of the earlier missions.

      No matter how fast current and future craft go and where, the more data points developed in the exceedingly tiny pinpoint slices taken of the interstellar medium, or galactic expanse, the better.

    • Robin Datta October 14, 2021, 10:57

      The interstellar medium could be the interhelion: in a Google search several instances of the word appear in science fiction, although it is not found (yet) in dictionaries.

      • mick October 18, 2021, 5:10

        How about “extrasolar”?

    • Shaun October 29, 2021, 15:27

      Perhaps we should call it Steorrasae? (Starsea)

  • Ron S. October 13, 2021, 9:53

    I have mixed feelings about these long duration science missions. I agree that the scientific value is significant and valuable. However, that value is almost certainly irrelevant to interstellar travel.

    In particular, we do not need further detailed knowledge of the ISM to launch an *exploration* mission. It’s effectively a vacuum and not an impediment. We have other large challenges to overcome before we go interstellar.

    “shift in perspective in turn feeds the interstellar ambition”

    I have my doubts with respect to the broader society. We have a short attention span. Our long duration missions are routinely overlooked and forgotten, other than the occasional reminder when there is a science result to report. Yet the enthusiasm of society is needed before major investment in an interstellar project is probable.

    “lines up well enough with the ecliptic to offer a boost from the angular momentum available to a departing craft”

    Yes, but is it enough to matter? Quibbling over 30 km/s highlights how limited and unproductive our existing and technologically feasible propulsion options remain with respect to the high velocities required for effective interstellar travel. I think the quibbling and reluctance will necessarily continue until we get lucky (if that is possible) with new physical mechanisms for propulsion.

    • Alex Tolley October 13, 2021, 13:13

      The longevity of the mission is handled by intermediate mission goals to ensure there remains incentive and interest to do these ISM missions. Just as the primary mission of the Voyagers was planetary and their longevity has given us the bonus of some ISM science.

      To use the boat analogy again, we should keep building and developing canoes, and inshore sailboats to explore the coasts even though it might be centuries before true ocean capable ships could be built. It may be that we are limited to less than 0.01c, but if we can build large space habits then that velocity might be acceptable. KSR’s cautionary novel “Aurora” has the starship built around the habitat principle, capable of just 0.1c that makes its more than century, multi-generation journey to Tau Ceti 12 ly away (and return). Could the journey be made that takes 1200 years or more given the advanced technology we could have?

      Regarding the ISM density. While it is for all intents and purposes a vacuum for our probes, it won’t be if we have probes and ships able to travel at relativistic velocities. Then we will need maps to avoid the equivalent dangers to “reefs” and “storms”. Starships may need to plan for more circuitous routes to maximize safety. Starship designers and shipping companies will be glad that maps are available so that the hazards are known in advance.

      • Ron S. October 13, 2021, 22:21

        “less than 0.01c”

        Well, that’s 3000 km/s. In the context of my comment, 30 km/s is peanuts in comparison when we’re discussion propulsion option.

        “we will need maps to avoid the equivalent dangers to “reefs” and “storms””

        Yes, but we already have rough maps of the ISM in the vicinity of the heliosphere (it’s right there in Paul’s article), and if we do go fast enough it will matter. These probes will do important science but will likely not tell us much more of importance with regard to fast travel.

    • Antonio October 13, 2021, 15:58

      “It’s effectively a vacuum and not an impediment.”

      Nope. A ship travelling to another star needs a very good shield to protect itself from dust and particles it mops out in its journey. And, since mass is paramount for a spaceship, and even more so for a starship, the better we know the ISM, the better we can adjust the mass of the shield.

      “Our long duration missions are routinely overlooked and forgotten, other than the occasional reminder when there is a science result to report. Yet the enthusiasm of society is needed before major investment in an interstellar project is probable.”

      In the paper there are some examples of long human endeavours, like building the cathedrals, but also scientific endeavours, like CERN, created in the 1950s and still alive and healthy. Indeed, particle physicists routinely plan for new accelerators decades in advance, and then run them for decades while planning for the next.

      “I think the quibbling and reluctance will necessarily continue until we get lucky (if that is possible) with new physical mechanisms for propulsion.”

      I think the problem is mostly political, not physical. If there were political will to do it, we could build and launch a ship to Alpha Centauri in a couple of decades, using technology like pulsed fission (Orion) or laser sails.

      • Ron S. October 13, 2021, 22:26

        “A ship travelling to another star needs a very good shield to protect itself from dust and particles it mops out in its journey.”

        Since you’re the second person to misunderstand my point, perhaps I did not express myself well. Rereading what I wrote, yes, I was not clear.

        Yes, I agree with you. My intended point was that we already know enough about the ISM (an “almost” vacuum but still a danger) to make plans for spacecraft protection. An incremental improvement of our knowledge of the ISM composition really doesn’t change what must be done.

  • ljk October 13, 2021, 10:37


    How Far Have the Voyagers Got? What Happened to Them?

    October 11, 2021

    Roughly half a year before this video was posted, on 15 April 2021, the automatic space probe New Horizons became the fifth spacecraft in the history of the humanity to go beyond the point of 50 AU from the Sun. It was the Voyagers that had crossed this mark before, with the probes Pioneer 10 and Pioneer 11 the first ever to do so. None of these space wanderers are likely to ever return to the Earth. With some of them still active on their missions, others have gone quiet forever…

    ➥ Patreon – https://www.patreon.com/off_kosmo

    00:00 Intro
    01:28 New Horizons
    04:29 Pioneer 10
    05:31 Pioneer 11
    06:41 Voyager 1
    09:29 Voyager 2
    11:32 Ending

  • Jeff Wright October 14, 2021, 4:50

    I think ‘cis-solar space’ works to the Oort…”Post Littoral Envelope Area” outside the Oort.

    Proximan Approaches…Proximan Littoral Zone.

    “Deep Interstellar” between galactic arms